Composite ceramic powder, method for manufacturing the powder, electrode for solid electrolytic fuel cell, and method for manufacturing the electrode

ABSTRACT

An object of the present invention is to provide composite ceramic powder containing composite ceramic particulates as constituent particulates. Each of the composite ceramic particulates is constituted of a group of first particles and a group of second particles in which the first particles are localized around the second particles. A spray pyrolysis is used to localize the first particles around the second particles, thereby producing such composite ceramic particulates.

TECHNICAL FIELD

The present invention relates to composite ceramic powder and a methodof manufacturing the same, and electrodes of a solid oxide fuel cell(hereinafter referred to as "SOFC") and a method of manufacturing thesame.

BACKGROUND ART

In general, composite ceramic powder containing two or more types ofceramic materials is constituted of secondary particles or compositeparticulates each of which is formed by agglomerating primary particlesmade of respective ceramic materials. A conventional method ofmanufacturing such composite ceramic powder includes the steps ofproviding two or more types of coarse powdered materials, pulverizingthe coarse powdered materials by using a pulverizer to form finepowdered materials, mixing the produced fine powdered materials in aball mill, calcining the mixture, and milling the calcined mixture toform composite ceramic particulates. Another conventional method ofmanufacturing such composite ceramic powder includes the steps ofdissolving two or more types of powdered materials to form a solution ofthe materials, thermally decomposing by dropping the solution into afurnace, calcining the decomposed materials, and milling the calcinedmaterials to form fine composite ceramic particulates. The latter methodis known as a drip pyrolysis.

Some composite ceramic powder thus manufactured has been used as amaterial for forming electrodes of an SOFC.

In the composite ceramic powder thus manufactured, the composite ceramicparticulates have irregular shapes. This is because they are produced bymechanically grinding mass products which are formed by agglomeratingthe primary particles made of respective materials. Thus, it issubstantially impossible to obtain the composite ceramic particulateshaving spherical shapes. Moreover, the electrodes of the SOFC made ofsuch composite ceramic powder do not exhibit acceptable electricalproperties.

The present inventors have developed composite ceramic powderconstituted of spherical composite particulates and a method ofmanufacturing the same. The developed technique is fully shown inJapanese Patent Application No. 6-82399 (Japanese Laid-Open PatentPublication No. 7-267613) previously filed.

As described therein, the composite ceramic particulates contained inthe composite ceramic powder are solid spherical particulates each ofwhich is produced by agglomerating the primary particles in asubstantially uniform dispersion state. Further, the method ofmanufacturing the composite ceramic powder constituted of such sphericalcomposite particulates includes the steps of atomizing a solution of twoor more types of raw materials to form mist thereof, drying the mist attemperatures below the temperature at which the raw materials can bethermally decomposed, and then thermally decomposing the raw materials.

This technique may provide the composite ceramic powder containing thespherical composite particulates each of which is produced byagglomerating the primary particles in the substantially uniformdispersion state. The composite ceramic powder thus manufactured mayexhibit increased electrical properties. Therefore, the compositeceramic powder is very suitable as the materials for forming theelectrodes.

In the composite ceramic powder thus manufactured, the primary particlesin each spherical composite particulate are substantially uniformlydispersed. In other words, each composite particulate has a uniformcomposite form. However, if each type of primary particles may exhibitinherent functions, it is not necessarily desirable that the compositeparticulate has such a uniform composite form.

It is desired to develop a new and superior composite form of thesecondary particles or composite ceramic particulates in which theprimary particles are preferably dispersed in a controlled dispersionstate, thereby further increasing properties of composite ceramic powderconstituted of such composite ceramic particulates and articles made ofthe composite ceramic powder and finding new uses of the compositeceramic powder.

Moreover, the electrodes of the SOFC formed of the conventionalcomposite ceramic powder gradually deteriorates with time and results ininferior performance. Such deterioration of the electrodes is caused byundesirable aggregation of the conductive materials derived from some ofthe primary particles. In general, the electrodes of the SOFC mustmaintain their performance for a long period of time such that the SOFCcan be continuously used over tens of thousands of hours.

To remove such a disadvantage, it is desired to change the compositeform of the composite ceramic particulates of the composite ceramicpowder for effectively preventing the undesirable aggregation of theprimary particles, so that the electrodes made of the composite ceramicpowder do not degrade for a long period of time.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide composite ceramicpowder constituted of composite ceramic particulates each having a newcomposite form and a method of manufacturing the same. Moreover, anotherobject of the present invention is to provide electrodes of an SOFCwhich may maintain electrical properties thereof and a method ofmanufacturing the same.

To achieve the objects described above, the present inventors providethe following inventions.

The present invention is directed to composite ceramic powder containingcomposite ceramic particulates as constituent particulates. Each of thecomposite ceramic particulates is constituted of a group of firstparticles and a group of second particles in which the first particlesare localized around the second particles.

With this invention, each composite ceramic particulate of the compositeceramic powder has a novel composite form in which the first particlesare localized around the second particles. This invention enables thefirst and second particles to be particles having different functionssuch that the composite ceramic powder and any ceramic products thereofmay have new additional functions and improved conventional functions.The composite ceramic powder is suitable for forming the electrodes ofthe SOFC, specifically for forming a fuel electrode of the SOFC. Thecomposite ceramic powder is also suitable for forming a sinteredcatalytic product. The electrodes formed by sintering such compositeceramic powder may stably exhibit desired electrical properties for along period of time.

The present invention provides a method of manufacturing compositeceramic powder containing composite ceramic particulates, each of thecomposite ceramic particulates being constituted of a group of firstparticles and a group of second particles in which the first particlesare localized around the second particles, which includes the steps ofatomizing a raw material liquid containing raw materials of the firstparticles and raw materials of the second particles, thereby formingmist thereof, drying the mist at temperatures below the temperature atwhich each raw material can be thermally decomposed, and thermallydecomposing the raw materials. In the step of drying the mist, the rawmaterials of the first particles are localized around the raw materialsof the second particles.

This method may produce the composite ceramic powder containing thecomposite particulates in which the first particles are localized aroundthe second particles. With this method, physical states of the rawmaterials in the atomized raw material liquid may be controlled in thestep of drying the mist, thereby controlling the composite form of eachcomposite particulate to be produced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view of a formation process of a compositeceramic particulate in a method of manufacturing composite ceramicpowder according to the present invention;

FIG. 2 is a schematic view of a spray pyrolysis machine used in thepresent invention;

FIG. 3 is a photograph (magnification of ×30000) of the compositeceramic powder taken by an electron microscope;

FIG. 4 is a photograph (magnification of ×10000) of a fuel electrode ofan SOFC (Ni--YSZ fuel electrode) made of the composite ceramic powdertaken by the electron microscope;

FIG. 5 is a fragmentary pictorial view of an encircled portion in FIG.4; and

FIG. 6 are graphs showing polarization potentials of the fuel electrodeof the SOFC vs. time.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described below in detail.

Composite ceramic powder of the present invention includes compositeceramic particulates each of which is constituted of a group of firstparticles and a group of second particles. With regard to each compositeceramic particulate, the second particles are agglomerated in acore-like pattern, and the first particles are agglomerated in ashell-like pattern to entirely cover the surface of the agglomeratedsecond particles. In other words, the first particles are localizedaround the agglomerated second particles and are not dispersed in thesecond particles. It is important to note, however, that the firstparticles may be agglomerated as another pattern. For example, they maybe agglomerated as an incomplete shell-like pattern so as to partiallycover the surface of the agglomerated second particles.

Each composite ceramic particulate may have various types of shapes.Each composite ceramic particulate may have an indeterminate shape, aspherical shape, a deformed spherical shape or other shapes. Accordingto a method of manufacturing the composite ceramic powder of the presentinvention, each composite ceramic particulate as produced has asubstantially spherical shape. As will be appreciated, the producedcomposite ceramic particulates may be ground to form indeterminate shapeof composite ceramic particulates in each of which the first particlesremain localized around the second particles.

In the present invention, it is preferable that each compositeparticulate has a spherical shape or a substantially spherical shape. Ifeach composite particulate has such a shape, it may present apoint-contact or uniform contact with the adjacent composite ceramicparticulates. The composite ceramic powder constituted of such compositeceramic particulates may be sintered with a good sintering state,thereby forming a uniform sintering structure. Additionally, suchcomposite ceramic powder may facilitate compacting or pressing and alsopermit close packing.

Each composite ceramic particulate may be of a solid type or a hollowtype. However, the solid type one is preferable in respect to strengthand electrical properties thereof. Moreover, each composite ceramicparticulate may have wrinkling on the surface thereof.

The first particles may be particles made of a single type of materialor a blend of particles made of two or more types of materials.Similarly, the second particles may be particles made of a single typeof material or a blend of particles made of two or more types ofmaterials.

In the present invention, it is preferable that the second particles aremade of catalytic materials which may exhibit catalytic activity. On theother hand, it is preferable that the first particles are made ofcarrier materials which may act as a catalytic carrier.

The composite ceramic powder containing the composite ceramicparticulates each of which is composed of the first and second particlesmade of such materials is suitable for forming a sintered catalyticproduct. In the sintered catalytic product produced by sintering suchcomposite ceramic powder, agglomerated catalytic particles aresurrounded by agglomerated carrier particles to form a dispersionstructure in which the agglomerated catalytic particles are separatelycarried by the agglomerated carrier particles. Such a structure mayprevent aggregation of the agglomerated catalytic particles. Accordingto a carrying structure of the agglomerated catalytic particles, theagglomerated catalytic particles are prevented from mutually bondingwhen the sintered catalytic product is used at higher temperatures. Thismay prevent aggregation of the agglomerated catalytic particles whichmay be caused by the mutual bonding thereof. As a result, theagglomerated catalytic particles maintain specific active surface areas,thereby preventing reduction of the catalytic activity thereof.

In the composite ceramic powder suitable for forming the sinteredcatalytic product, the materials for the second particles may be one ormore members selected from the group consisting of metals such asnickel, cobalt and iron; noble metals such as platinum, palladium andruthenium; and oxides such as zinc oxide, tin oxide, copper oxide andlanthanum manganate. Further, the materials for the first particles maybe one or more members selected from the group consisting of oxides suchas aluminum oxide, silicon oxide, magnesium oxide and titanium oxide.

Moreover, in the present invention, it is preferable that the secondparticles are made of precursor materials of conductive materials forelectrodes of an SOFC. On the other hand, it is preferable that thefirst particles are made of precursor materials of carrier materialseach of which may act as a carrier for the conductive materials in theelectrodes.

In the electrodes produced by sintering the composite ceramic powdercontaining the composite ceramic particulates each of which is composedof the first and second particles made of such precursor materials,agglomerated particles of the conductive materials or conductiveparticles are surrounded by the agglomerated particles of the carriermaterials or carrier particles to form a dispersion structure in whichthe agglomerated conductive particles are separately carried by thecarrier particles. Such a structure may prevent undesirable aggregationof the conductive particles. According to the structure of theelectrodes of the SOFC, the conductive particles are prevented fromaggregating with each other when the electrodes are used at highertemperatures. This may prevent the electrodes from deteriorating theelectrical properties thereof. Consequently, such composite ceramicpowder may provide the electrodes which may stably exhibit desiredelectrical properties for a long period of time.

In this specification, the conductive materials for the electrodes ofthe SOFC have conductive properties and catalytic properties.Additionally, the carrier materials for the electrodes of the SOFC havecarrying properties for carrying the conductive materials and ionicconductive properties.

In the composite ceramic powder suitable for forming a fuel electrode ofthe SOFC, the materials for the second particles may be one or moremembers selected from the group consisting of oxides of nickel, solidsolutions of oxides of nickel and oxides of magnesium, oxides of cobaltand ruthenium. Further, the materials for the first particles may be oneor more members selected from the group consisting of stabilizedzirconia (FSZ) represented by yttria stabilized zirconia (YSZ),partially stabilized zirconia (PSZ) and ceric oxide doped withrare-earth oxides. Preferably, the second particles and the firstparticles are made of NiO and YSZ, respectively.

In the composite ceramic powder suitable for forming an air electrode ofthe SOFC, the materials for the second particles may be one or moremembers selected from the group consisting of (La,Sr)MnO₃, (La,Ca)MnO₃,(La,Sr)CoO₃ and (La,Ca)CoO₃. Further, the materials for the firstparticles may be one or more members selected from the group consistingof stabilized zirconia, partially stabilized zirconia and ceric oxidedoped with rare-earth oxides. Preferably, the second particles and thefirst particles are made of (La,Sr)MnO₃ and YSZ, respectively.

(Method of Manufacturing Composite Ceramic Powder)

To manufacture the composite ceramic powder constituted of the compositeceramic particulates each having a composite form in which theagglomerated second particles are separately carried by the agglomeratedfirst particles, a spray pyrolysis is preferably used. The spraypyrolysis includes the steps of preparing a raw material liquidcontaining raw materials of the first particles and raw materials of thesecond particles, atomizing the raw material liquid by directingultrasonic waves or the like to form mist thereof, drying the mist attemperatures below the temperature at which each raw material can bethermally decomposed, that is, at temperatures at which none of the rawmaterials contained in the raw material liquid can be substantiallydecomposed, and then thermally decomposing the raw materials. The spraypyrolysis is described in "Ceramic Powder Synthesis by Spray Pyrolysis"by Gray L. Messing et al; Journal of the American Ceramic Society (1993,Vol. 76, No. 11, Pages 2707-2726).

It is important to note that the terminology "thermal decomposition" inthe present invention means chemical changes of substances, for example,oxidation of the substances and/or crystallization of amorphousmaterials. As will be appreciated, development of the crystallizationmay identified by an X-ray diffraction analysis of treated materialssince a crystallized material may form peaks in a diffracted spectrum ofan X-ray diffraction.

(Raw Materials for Composite Ceramic Powder)

The raw materials to produce the composite ceramic particulates may besolutions of various kinds of metallic salts or soils of various kindsof metallic oxides which are applicable to the spray pyrolysis.

Types of the raw materials correspond to the types of the first andsecond particles constituting the composite ceramic particulates.

Further, the raw materials are selected based on uses of the compositeceramic powder containing the composite ceramic particulates.

With the composite ceramic powder for forming the sintered catalyticproduct described above, the raw materials for the second particles arethose which may be thermally decomposed to produce the precursormaterials of the catalytic materials, and the raw materials for thefirst particles are those which may be thermally decomposed to producethe precursor materials of the carrier materials for carrying thecatalytic materials. It is to be noted that the "precursor material" ofthe catalytic materials in the present invention means any materialswhich may inherently exhibit the catalytic activity or which may exhibitthe catalytic activity when processed with sintering, reducing reactionor other treatment. Also, the "precursor materials" of the carriermaterials in the present invention means any materials which mayinherently act as the catalytic carriers or which may act as thecatalytic carriers when processed with sintering, reducing reaction orother treatment.

The raw materials suitable for the second particles are chemicalcompounds or combinations thereof which may be thermally decomposed toproduce the precursor materials of the catalytic materials, that is, oneor more members selected from the group consisting of metals such asnickel, cobalt and iron; noble metals such as platinum, palladium andruthenium; and metallic oxides such as zinc oxide, tin oxide, copperoxide and lanthanum manganate. Moreover, the raw materials suitable forthe first particles are chemical compounds or combinations thereof whichmay be thermally decomposed to produce the precursor materials of thecarrier materials, that is, one or more members selected from the groupconsisting of oxides such as aluminum oxide, silicon oxide, magnesiumoxide and titanium oxide.

With the composite ceramic powder for forming the electrodes of theSOFC, the raw materials for the second particles are those which may bethermally decomposed to produce the precursor materials of theconductive materials, and the raw materials for the first particles arethose which may be thermally decomposed to produce the precursormaterials of the carrier materials for carrying the conductivematerials. It is to be noted that the "precursor materials" of theconductive materials in the present invention means any materials whichmay inherently exhibit conductivity or which may exhibit conductivitywhen processed with sintering, reducing reaction or other treatment.Also, the "precursor materials" of the carrier materials in the presentinvention means any materials which may inherently act as the carriersor which may act as the carriers when processed with sintering, reducingreaction or other treatment. For example, when the conductive materialfor the electrodes of the SOFC and the precursor material thereof arenickel and oxides of nickel, respectively, the raw material may benickel acetate which may be thermally decomposed to produce oxides ofnickel.

With the composite ceramic powder for forming the fuel electrode of theSOFC, the raw materials suitable for the second particles are chemicalcompounds or combinations thereof which may be thermally decomposed toproduce the precursor materials of the conductive materials, that is,one or more members selected from the group consisting of (1) oxides ofnickel, (2) solid solution of oxides of nickel and oxides of magnesium,(3) oxides of cobalt and (4) ruthenium.

Specifically, the raw materials may be one or more members selected fromthe group consisting of acetate, nitrate, carbonate, oxalate and othersalts of nickel; acetate, nitrate, carbonate, oxalate and other salts ofmagnesium; acetate, nitrate, carbonate, oxalate and other salts ofcobalt; acetate, nitrate, carbonate, oxalate and other salts ofruthenium; and hydroxides of nickel, magnesium, cobalt and ruthenium.

On the other hand, the raw materials suitable for the first particlesare chemical compounds or combinations thereof which may be thermallydecomposed to produce the precursor materials of the carrier materials,that is, one or more members selected from the group consisting of (1)stabilized zirconia (FSZ) represented by yttria stabilized zirconia(YSZ), (2) partially stabilized zirconia (PSZ) and (3) ceric oxide dopedwith rare-earth oxides.

Specifically, the raw materials may be one or more members selected fromthe group consisting of soils of FSZ; soils of PSZ; sols of ceric oxidedoped with rare-earth oxides; acetate, nitrate, carbonate, oxalate andother salts of the elements constituting the FSZ, PSZ or ceric oxidedoped with rare-earth oxides; and hydroxides of the elementsconstituting the FSZ, PSZ or ceric oxide doped with rare-earth oxides.

With the composite ceramic powder for forming the air electrode of theSOFC, the raw materials suitable for the second particles are chemicalcompounds or combinations thereof which may be thermally decomposed toproduce the precursor materials of the conductive materials, that is,one or more members selected from the group consisting of (La,Sr)MnO₃,(La,Ca)MnO₃, (La,Sr)CoO₃ and (La,Ca)CoO₃.

For example, such raw materials may be one or more members selected fromthe group carbonate,g of acetate, nitrate, carbonate, oxalate and othersalts of lanthanum; acetate, nitrate, carbonate, oxalate and other saltsof strontium; acetate, nitrate, carbonate, oxalate and other salts ofmanganese; acetate, nitrate, carbonate, oxalate and other salts ofcalcium; acetate, nitrate, carbonate, oxalate and other salts of cobalt;and hydroxides of lanthanum, strontium, manganese, calcium and cobalt.

On the other hand, the raw materials suitable for the first particlesare chemical compounds or combinations thereof which may be thermallydecomposed to produce the precursor materials of the carrier materials,that is, one or more members selected from the group consisting of FSZ,PSZ and ceric oxide doped with rare-earth oxides.

For example, the raw materials may be one or more members selected fromthe group consisting of sols of FSZ; sols of PSZ; sols of ceric oxidedoped with rare-earth oxides; acetate, nitrate, carbonate, oxalate andother salts of the elements constituting the FSZ, PSZ or ceric oxidedoped with rare-earth oxides; and hydroxides of the elementsconstituting the FSZ, PSZ or ceric oxide doped with rare-earth oxides.

Some of the raw materials are mixed with a solvent to formulate the rawmaterial liquid. The raw material liquid may be a solution formed bydissolving the raw materials in the solvent, a sol formed by dispersingthe raw material in the solvent, or a mixture thereof. As will beappreciated, the raw materials may be in a dissolved state or a solidstate in the raw material liquid.

As described hereinafter, when the raw material liquid is dried, it ispossible to produce mist of the raw material liquid in which the firstparticles are localized around the surface of the second particles or inwhich the first and second particles are uniformly dispersed, dependingon the difference between the raw materials of the first particles andthe raw materials of the second particles in solubility or physicalstates in the raw material liquid.

(Atomization, Drying and Thermal Decomposition of the Raw MaterialLiquid)

Subsequently, the raw material liquid is atomized. When the raw materialliquid is atomized by directing the ultrasonic waves to form the mistthereof, frequencies of a transducer for generating the ultrasonic wavesmay be preferably controlled to change the size of the composite ceramicparticulates to be ultimately produced.

The formed mist is then dried at temperatures below the temperature atwhich each raw material can be thermally decomposed, that is, attemperatures at which none of the raw materials contained in the rawmaterial liquid can be substantially decomposed. Thereafter, the rawmaterials are thermally decomposed.

In the raw material liquid as formulated, if each of the raw materialsof the first particles has solubility in the solvent greater than thatof each of the raw materials of the second particles, the raw materialsof the second particles each having lower solubility may precipitate andagglomerate before the raw materials of the first particles precipitateand agglomerate when the mist is dried. As a result, the first particlesare localized around the surface of the second particles.

Further, in the raw material liquid, if each of the raw materials of thefirst particles is in the solid state whereas each of the raw materialsof the second particles is in the dissolved state, the raw materials ofthe first particles in the solid state tend to concentrate to thesurface of the mist when the mist is dried. As a result, the rawmaterials of the second particulates in the dissolved state precipitateand agglomerate in the central portion of the mist so that the firstparticles are localized around the surface of the second particles.

Additionally, the tendency of the formation of the composite form inwhich the first particles are localized around the second particlesincreases as the total concentration of the raw materials in the rawmaterial liquid becomes greater.

Moreover, a suitable range of the concentration (mol %) of each of theraw materials and a suitable range of the total concentration of the rawmaterials may be determined depending on the combination of the rawmaterials, the physical states of the raw materials in the raw materialliquid, that is, whether each of the raw materials is in the dissolvedstate or the solid state in the raw material liquid, or otherconditions.

The following is an example for forming the fuel electrode of the SOFCin which the sol of YSZ (average particle size of YSZ particles: 60 nm)and nickel acetate tetrahydrate (aqueous solution) are used as the rawmaterial of the first particles and the raw material of the secondparticles, respectively. These raw materials are mixed to formulate theraw material liquid. The raw material liquid thus formulated is thenthermally decomposed by the spray pyrolysis, thereby to produce thecomposite particulates. In each composite particulate, the particles ofNiO are centrally agglomerated and the particles of YSZ are positionedaround the agglomerated NiO particles. The composite particulates aresintered to form the fuel electrode of the SOFC. In the raw materialliquid, the proportion of nickel to YSZ is preferably 90:10 to 50:50 inmol %. This is because the YSZ contents greater than 50 mol % may reducethe conductivity of the fuel electrode of the SOFC and because the YSZcontents lesser than 10 mol % may not sufficiently carry or supportnickel in the fuel electrode. More preferably, the proportion of nickelto YSZ in mol % is 80:20 to 70:30.

Furthermore, the concentration of nickel acetate in the raw materialliquid is preferably greater than 0.01 mol/l, more preferably, 0.2-0.3mol/l, as a thermal decomposition product thereof or nickel oxide (NiO).

When the mist is dried and thermally decomposed, the mist iscontinuously moved by a carrier gas. Increased flow rates of the carriergas may contribute to the formation of the hollow spherical particulatesor the enlargement of a hollow space of each particulate. On thecontrary, reduced flow rates of the carrier gas may contribute to theformation of the solid spherical particulates or the reduction of thehollow space of each particulate. In other words, inner forms of thespherical particulates to be produced can be effectively controlled bychanging the flow rates of the carrier gas.

Also, when the mist is dried and thermally decomposed, highertemperature gradients may contribute to the formation of the hollowparticulates or the enlargement of a hollow space of each particulate.On the contrary, lower temperature gradients may contribute to theformation of the solid spherical particulates or the reduction of thehollow space of each particulate. In other words, the inner forms of thespherical particulates to be produced can be controlled by changing thetemperature gradients between the drying process and the thermallydecomposing process.

Thus, the composite particulates can be produced by atomizing, dryingand thermally decomposing the raw material liquid. A machine suitablefor producing the composite ceramic particulates includes an atomizingmeans for atomizing the raw material to form mist, a moving passage formoving the mist, a plurality of heat generating means positioned in thepassage and directed in the direction in which the mist is moved. Withthe machine, the mist is dried and thermally decomposed while moving inthe moving passage.

Such a machine is exemplary shown in FIG. 2. The machine 1 has anatomizing chamber 2 in which a transducer 3 as the atomizing means isprovided, a silica glass tube 5 as the moving passage communicating withthe atomizing chamber 2, electric heat generators 7A, 7B, 7C and 7Darranged on the outside surface of the silica glass tube 5 to heat theinterior of the silica glass tube 5, and an electric dust collector 8coupled to the silica glass tube 5. A carrier gas is introduced into theatomizing chamber 2 so that the mist M flows through the silica glasstube 5.

Each of the electric heat generators 7A, 7B, 7C and 7D is controlledsuch that the mist M is dried and thermally decomposed while moving inthe silica glass tube 5.

The raw materials contained in the mist M are oxidized and/orcrystallized to produce the composite particulates CP after they aredried and thermally decomposed. The composite particulates CP thusproduced are, for example, collected by the electric dust collector. Thecollected composite particulates are then calcined and crystallized.Thereafter, the calcined composite particulates are milled and unboundto form the composite ceramic powder.

In the composite ceramic powder, as shown in FIG. 1, each compositeparticulates CP is constituted of the agglomerated second particles P2and the agglomerated first particles P1 which is localized on thesurface of the agglomerated second particles P2. Therefore, the surfaceof the agglomerated second particles P2 is covered with the firstparticles P1. It is to be noted, however, that the surface of theagglomerated second particles P2 may be partially covered with the firstparticles P1.

According to the method, the composite particulates each having thespherical shape can be easily obtained. Such composite particulates maybe further ground to form the indeterminate shape of composite ceramicparticulates.

For example, if the raw materials are selected for forming the sinteredcatalytic product described above, the composite ceramic powder suitablefor forming the sintered catalytic product can be produced. Thecomposite ceramic powder contains the composite particulates each ofwhich is constituted of the first particles made of the precursormaterials of the carrier materials and the second particles made of theprecursor materials of the catalytic materials which may exhibit thecatalytic activity when preferably treated.

Further, if the raw materials are selected for forming the electrodes ofthe SOFC described above, the composite ceramic powder suitable forforming the electrodes of the SOFC can be produced. The compositeceramic powder contains the composite particulates each of which isconstituted of the first particles made of the precursor materials ofthe carrier materials which may support the conductive materials and thesecond particles made of the precursor materials of the conductivematerials.

When the composite ceramic powder containing the composite particulateseach having such a composite form is sintered, particle growth of thefirst and second particles may occur during the course of the sinteringprocess. As a result, each composite particulate is deformed. Furtherprogress of the particle growth of the first and second particles mayform the dispersion structure in which an indeterminate shape of mass ofthe agglomerated second particles is surrounded and separately supportedby an indeterminate shape of mass of the agglomerated first particles.

Such a dispersion structure may provide a new supporting structure ofthe catalytic particles and a novel structure of the electrodes of theSOFC.

Particularly, when the composite ceramic powder suitable for forming thesintered catalytic product as described above is sintered, the particlegrowth of the first and second particles in each composite particulatemay occur. As a result, the first and second particles may form theindeterminate shape of mass of the agglomerated carrier particles andthe indeterminate shape of mass of the agglomerated catalytic particles,respectively, thereby forming the dispersion structure in which theagglomerated catalytic particles are surrounded and separately supportedby the agglomerated carrier particles. The catalytic particles thuscarried may constantly exhibit the catalytic activity.

The sintered catalytic product formed by sintering the composite ceramicpowder is treated to provide the catalytic activity to the catalyticparticles, if necessary. Specifically, the sintered catalytic product isprocessed with sintering, sintering accompanied by reduction treatmentor other treatments which are conducted as separate processes.

Further, when the composite ceramic powder suitable for forming theelectrodes of the SOFC as described above is sintered, the electrodes ofthe SOFC are formed. In the electrodes, the indeterminate shape of massof the agglomerated conductive particles is surrounded and separatelycarried by the agglomerated carrier particles. According to thestructure of the electrodes of the SOFC, the agglomerated conductiveparticles are effectively prevented from aggregating with each otherwhen the electrodes are used at higher temperatures. This may preventthe electrodes from deteriorating the electrical properties thereof. Aswill be appreciated, the electrodes are then processed with thereduction treatment or other treatments to provide the conductivity tothe conductive particles, if necessary.

When the composite ceramic powder including the composite particulateseach of which is constituted of the first particles made of YSZ and thesecond particles made of NiO is sintered to form a sintered product, theagglomerated NiO particles are surrounded and separately carried by theagglomerated YSZ particles. If the sintered product is treated to reduceNiO to Ni and is used as one of the electrodes of the SOFC, the Niparticles are effectively prevented from bonding and/or aggregating witheach other. Therefore, the sintered product thus treated may stablyexhibit the desired electrical properties for a long period of time.This is preferable for the fuel electrode of the SOFC.

(Example)

An example of the present invention will be described in detail. It isto be noted, however, that the following example should not be construedas limiting the invention.

In this example, the composite ceramic powder suitable for forming thefuel electrode of the SOFC was manufactured. The composite ceramicpowder contains the composite particulates each of which is constitutedof the particles of NiO and the particles of YSZ. As will beappreciated, the NiO particles are particles of the precursor materialsof the conductive materials (Ni) and correspond to the second particlesof the composite particulates. On the other hand, the YSZ particles areparticles of the precursor materials of the carrier materials andcorrespond to the first particles of the composite particulates.

Ni(CH₃ COO)₂ ·4H₂ O (reagent grade) as the raw materials for the NiOparticles and the sol of YSZ (reagent grade) as the raw materials forthe YSZ particles were dissolved to formulate the raw material liquid.The proportion of Ni(CH₃ COO)₂ ·4H₂ O to the sol of YSZ were determinedsuch that the proportion of the thermal decomposition product (oxide) orNiO to YSZ was 75:25 in mol %. Also, the concentration of Ni(CH₃ COO)₂·4H₂ 0 in the raw material liquid was 0.25 mol/l as NiO.

Ni(CH₃ COO)₂ was dissolved in the raw material liquid to produce Ni ionstherein. On the other hand, YSZ was in the solid state therein. Further,the solvent as used was water.

The raw material liquid was treated by using the machine (which wasmanufactured by Iwatani Sangyo Kabushiki Kaisha) shown in FIG. 2,thereby manufacturing the composite ceramic powder.

First, the raw material liquid was introduced into the atomizing chamber2 of the machine 1, with the transducer 3 energized to generate theultrasonic wave of 1.7 MHz, thereby producing the mist M of the rawmaterial liquid. The produced mist M was transferred by the carrier gasor air (flow rate: 3 l/min), and was introduced into the silica glasstube 5 previously heated by the electric heat generators 7A, 7B, 7C and7D. The electric heat generators 7A, 7B, 7C and 7D were controlled insuch a way that the interior of the silica glass tube 5 was heated to200° C., 400° C., 800° C. and 1000° C.

Thus, the mist M was slowly heated in the silica glass tube 5. As aresult, the mist M was dried at temperatures at which the raw materialscan not be thermally decomposed, and was then thermally decomposed tointermediate powder.

The intermediate powder discharged from the silica glass tube 5 providedwith the electric heat generators 7A, 7B, 7C and 7D was collected by theelectric dust collector 8. The collected intermediate powder wascalcined and recrystallized at 1000° C. for four hours. Thereafter, thecalcined powder was gently pulverized and unbound to the ultimatecomposite ceramic powder.

The composite ceramic powder was observed by a scanning electronmicroscope (SEM). As shown in FIG. 3, the composite ceramic powdercontains the spherical composite particulates. In each compositeparticulate, the surface of the agglomerated NiO particles is covered bythe agglomerated YSZ particles. In particular, as shown in the drawing,the agglomerated NiO particles having a relatively smooth surface iscovered by the agglomerated YSZ particles having a rough surface.Further, NiO and YSZ are qualitatively identified by utilizing an energydispersive spectroscopy (EDS).

The composite particulates have limited size variations and the averagesize thereof is about 1 μm. Each composite particulate is of the solidtype.

The composite ceramic powder was screen-stenciled on the surface of a10YSZ plate (ZrO₂ stabilized by 10 mol % Y₂ 0₃) having a thickness of0.5 mm. The plate was then heated at 1350° C. for two hours to bake thepowder on the surface thereof, thereby forming the fuel electrode havingan effective area of 10 cm². To screen-stencil the composite ceramicpowder on the plate, the composite ceramic powder was mixed withpolyethylene glycol of 0.4 gram as a binding agent and ethanol of 6 gramas a dispersing agent and was agitated in an automatic alumina mortarfor fifteen minutes to volatilize ethanol. Thereafter, the mixture wasscreen-stenciled (mesh #200) on the YSZ plate. The product was used inthe SOFC to reduce NiO to Ni, thereby forming the fuel electrode orNi/YSZ fuel electrode having a thickness of 30 μm.

A structure of the fuel electrode thus formed observed by SEM is shownin FIG. 4. Further, a pictorial view of an encircled portion in FIG. 4is shown in FIG. 5. When the composite ceramic powder is baked to sinterthe NiO particles and the YSZ particles, each composite particulate isdeformed, thereby producing the indeterminate shape of mass of theagglomerated NiO particles and the indeterminate shape of mass of theagglomerated YSZ particles. In particular, the indeterminate shape ofmass of the agglomerated YSZ particles is significantly formed by theparticle growth of the YSZ particles positioned on the surface of eachcomposite particulate.

In the fuel electrode used in the SOFC or processed with the reductiontreatment, the particle growth of Ni and YSZ occurs, thereby forming thedispersion structure in which Ni is surrounded by YSZ, that is, as shownin FIGS. 4 and 5, the large and dark colored mass of the agglomerated Niparticles is surrounded by the small and light colored mass of theagglomerated YSZ particles.

Moreover, in this example the composite ceramic powder suitable forforming the air electrode of the SOFC was manufactured. The compositeceramic powder contains the composite particulates each of which isconstituted of the particles of La(Sr)MnO₃ and the particles of YSZ.Like the fuel electrode, the composite ceramic powder was mixed withpolyethylene glycol and ethanol and was agitated to volatilize ethanol.Thereafter, the mixture was screen-stenciled on the YSZ plate. Theproduct was then heated at 1200° C. for four hours to bake the powder onthe surface thereof, thereby forming the air electrode. Additionally, aPt wire was bonded to the side surface of an additional YSZ plate toform a control electrode. These YSZ plates were tested by continuouslypassing electricity (current density: 300 mA/cm²) therethrough at 1000°C. for one thousand hours. Polarization potentials of the fuel electrodewith time were determined to evaluate long-term stability of theelectrode. The polarization potentials of the fuel electrode weredetermined by a current interrupter method. The determination resultsare shown in FIG. 6.

As will be recognized, the current interrupter method is a method inwhich current traveling through the cell is momentarily interrupted toinduce the change in voltage, thereby determining the polarizationpotentials following the change in voltage. With this method, thepolarization potentials of the electrode can be obtained aselectrochemical polarization potentials (η polarization potentials)which correspond to performance of the electrode and ohmic losses (IRpolarization potentials) which reflect resistance of electrolytes. It isto be understood that the electrode exhibiting smaller η polarizationpotentials may serve as a more efficient electrode.

As shown in FIG. 6, the electrochemical polarization potentials (ηpolarization potentials) are constant, that is, no apparent degradationof the electrode is observed over one thousand hours. This means thatthe electrode is stable over this period of time in microstructures ofthe three-phase interfaces (Ni--YSZ-fuel gas(H₂ +3%H₂ O) interfaces)which act as electrode reaction sites. In other words, the fuelelectrode in this example has the microstructures in which theagglomerated Ni particles are enclosed by the agglomerated YSZparticles, so that the aggregation of the agglomerated Ni particles iseffectively prevented.

Further, the polarization potentials of the electrode in this exampleare smaller than those of the fuel electrode formed of the compositeceramic powder which is manufactured by conventional methods such as apowdered materials mixing method and a dropping thermal decompositionmethod. This shows that the fuel electrode of the present invention isan efficient electrode.

The IR polarization potentials are reduced with time. This is becausecontact resistance between a nickel felt used as a collector and theNi/YSZ fuel electrode is improved.

Although the electrodes were additionally tested by continuously passingelectricity (current density: 300 mA/cm²) therethrough for threethousands hours, the electrochemical polarization potentials are notlowered.

Industrial Applicability

The composite ceramic powder according to the present invention can beused as materials for forming ceramic products such as the sinteredcatalytic product, the electrodes of the SOFC and other sintered ceramicproducts.

The method of manufacturing the composite ceramic powder according tothe present invention may provide the composite ceramic materials (sic)suitable for forming such ceramic products.

What is claimed is:
 1. Composite ceramic powder comprising compositeceramic particulates as constituent particulates, each of said compositeceramic particulates being constituted of a group of first particles anda group of second particles in which said first particles are localizedaround said second particles.
 2. The composite ceramic powder as definedin claim 1, wherein said second particles comprise particles made ofmaterials which may exhibit catalytic activity, and wherein said firstparticles comprises particles made of materials which may act as acatalytic carrier.
 3. The composite ceramic powder as defined in claim1, wherein said second particles comprise particles made of precursormaterials of conductive materials for electrodes of a solid oxide fuelcell, and wherein said first particles comprises particles made ofprecursor materials of carrier materials which may act as a carrier forthe conductive materials in the electrodes.
 4. The composite ceramicpowder as defined in claim 1, wherein said second particles compriseparticles made of nickel oxide (NiO), and wherein said first particlescomprises particles made of yttria stabilized zirconia.
 5. The compositeceramic powder as defined in claim 1, wherein said composite ceramicparticulates have substantially spherical shapes.
 6. A method ofmanufacturing an electrode of a solid oxide fuel cell, comprising thesteps of:sintering composite ceramic powder containing composite ceramicparticulates, each of the composite ceramic particulates beingconstituted of particles made of precursor materials of conductivematerials for the electrode and particles made of precursor materials ofcarrier materials which may act as a carrier for the conductivematerials in the electrode, in which the latter particles are localizedaround the former particles.
 7. An electrode comprising nickel particlessurrounded and separately supported by yttria stabilized zirconiaparticles, in a nickel:yttria stabilized zirconia molar ratio between90:10 and 50:50.
 8. A solid oxide fuel cell comprising a fuel electrodeas defined in claim 7 and an air electrode.
 9. A method of manufacturinga composite ceramic powder comprising the steps of:atomizing a liquidcomprising a first particle raw material and a second particle rawmaterial to form a mist, the first particle raw material and the secondparticle raw material each having a known thermal decompositiontemperature; drying the mist at a temperature below the thermaldecomposition temperature of both of the raw materials, to formparticulates in which the first particle raw material is disposed aroundthe second particle raw material; and thermally decomposing the firstparticle raw material and the second particle raw material in theparticulates.
 10. A method as in claim 9, wherein the first particle rawmaterial comprises nickel and the second particle raw material comprisesyttria stabilized zirconia, and wherein the liquid comprises the nickeland yttria stabilized zirconia in a nickel:yttria stabilized zirconiamolar ratio between 90:10 and 50:50.
 11. A method as in claim 9, whereinthe first particle raw material is in a solid state suspended in theliquid and the second particle raw material is dissolved in the liquid.12. A method as in claim 9, wherein the first particle raw material andthe second particle raw material are both dissolved in the liquid andthe first particle raw material has a solubility in the liquid that isgreater than the second particle raw material.
 13. Composite ceramicpowder comprising composite ceramic particulates, each of said compositeceramic particulates comprising a group of first particles and a groupof second particles in which said second particles form a core having asurface and said first particles are disposed at least partiallycovering the surface of said core.
 14. The composite ceramic powder ofclaim 13, wherein said second particles comprise a material havingcatalytic activity, and wherein said first particles comprise a materialwhich acts as a catalytic carrier.
 15. The composite ceramic powder asdefined in claim 13, wherein said second particles comprise a precursorof a conductive material for electrodes of a solid oxide fuel cell, andwherein said first particles comprise a precursor of a carrier materialfor the conductive material in the electrodes.
 16. The composite ceramicpowder as defined in claim 13, wherein said second particles comprisenickel oxide (NiO), and wherein said first particles comprise yttriastabilized zirconia.
 17. A method of manufacturing an electrode of asolid oxide fuel cell, comprising the steps of:sintering compositeceramic powder containing composite ceramic particulates, each of thecomposite ceramic particulates comprising particles made of a precursorof a conductive material for the electrode and particles made of aprecursor of a carrier material for the conductive material in theelectrode, in which the carrier material precursor particles aredisposed around the conductive material precursor particles.